By Darren Pemberton – Senior Associate Engineer
The mining industry has long acknowledged the closure, capping and rehabilitation of coal tailings deposits as being problematic.Characteristics of Coal Tailings and Soft Tailings Deposits
Tailings produced from coal handling and preparation plants (CHPP) in Australia typically have a low soil particle density and a high percentage of clay and silt fines. These types of materials have poor consolidation properties.
Soft coal tailings deposits arise as a consequence of the combination of the fundamental consolidation and strength properties of coal fines material, and the geometry of tailings storages that result in high rise rates – which in the case of in-pit tailings storages, can be in excess of 10 m/year.
High rise rates, which limit the potential for evaporative drying, in combination with a tailings material exhibiting poor consolidation characteristics, will ultimately result in a tailings deposit with low densities, and hence low shear strength characteristics. In worst case examples, the tailings can exhibit the properties of a liquid.
It can take many years (>10) for the top surface of soft tailings to dry and consolidate to a state that will support direct access by earthmoving equipment to facilitate safe placement of fill materials for closure and rehabilitation purposes.
Planning for closure during design and operation will save time and money
For more than 40 years, ATC Williams has been assisting clients in developing tailings management techniques that enhance the opportunity for safe future placement of capping fill materials and reducing capping closure costs. We encourage our clients to take a holistic view to tailings management and consider deposition practices with rehabilitation in mind.
When it comes to capping and closure of the facility, significant time and cost can be saved through the implementation of carefully considered tailings deposition practices that facilitate the development of a deep surface crust of tailings, increase density and minimise water table development.
There can be significant differences in coal tailings (or fine rejects) properties. These differences occur both within a single mine and from site to site as a result of seam differences, mining methods, and processing methods.
With such a diverse range of properties, a tailings deposition management solution that works for one site, will not necessarily work for another. A unique site-specific management solution will consider the particular tailings characteristics as well as other site constraints.
ATC Williams has extensive knowledge in tailings management practices and can tailor solutions to site-specific characteristics to meet future closure and rehabilitation objectives.
A safe capping design relies on a thorough understanding of the deposited tailings
The capping techniques that are employed and the time in which they can be conducted post-deposition are mostly dependent upon the shear strength profile achieved prior to capping placement, and the pore pressure in the tailings.
The estimation of the shear strength profile is arguably the most critical factor when considering the design of a capping system for a soft tailings deposit. It is essential to obtain the most accurate strength data possible through a thorough investigation to develop a safe and stable capping design.
There are many different methods for measuring the undrained shear strength of soft coal tailings deposits. These include shear vane testing, variable energy dynamic cone penetrometer (DCP) testing and cone penetrometer testing with pore pressure measurement (CPTu).
CPTu testing is generally considered the best method for obtaining data with a high level of accuracy. The CPTu has the crucial advantage of providing reliable and repeatable data to significant depths and gives a direct measure of pore pressure data. ATC Williams was one of the first to perform CPTu testing over soft coal tailings deposits more than twenty years ago and have completed many investigations in this manner since this time.
Every site has unique attributes, and as such, there is not a ‘one size fits all’ approach that can be applied to the design of a testing regime. Investigation techniques may require a combination of different methods, and it is the responsibility of the engineering practitioner to determine the most suitable method of investigation for a particular site.
Capping Methods for Closure Development
The primary objective of closure and rehabilitation of a coal tailings storage facility is to provide a final landform that is inherently stable both in terms of erode-ability and overall stability. To achieve a stable final landform, capping layers need to be placed over the tailings in a safe and controlled manner.
Capping methods utilised for a particular tailings storage facility will depend upon several factors, including:
- existing shear strength and pore pressure profiles in the deposited tailings
- existing tailings beach slope
- proposed final land use
- the time frame in which the storage is to be capped.
Uncontrolled placement of fill over a tailings dam will likely cause failures and significant displacement of the existing tailings, and this method is no longer favoured.
ATC Williams has designed over forty capping projects of soft coal tailings deposits throughout Australia utilising the following capping methods:
i. Placed capping layers (conventional capping);
ii. Placed capping layers with reinforcement;
iii. Radial Telescopic Conveyor Capping;
iv. Beached capping layers; and
v. Placement of secondary flocculated tailings.
These methods are described below, together with commentary on their relative suitability for application to coal tailings storage facilities.
Placed Capping Layers (Conventional Capping)
Where the strength of the tailings permits, it is possible to place an initial, relatively thin layer of fill using lightweight equipment (e.g. low ground pressure dozers, or smaller purpose-built machines).
This layer provides additional confinement and strength gain in the tailings and support for subsequent layers. Placement of this first layer is critical and is illustrated in Diagram 1 below.
Placed Capping Layers with Reinforcement
Where the strength of the tailings alone does not permit the placement of thin layers of fill material, reinforcing layers can be used to increase the bearing capacity of the tailings. Reinforcement materials may comprise woven geofabric, geogrids, or various combinations (geocomposites) of these. These materials provide tensile reinforcement in the capping layer and improve the stability, particularly local stability.
The technique for stabilisation of very low strength soil with reinforcement material is illustrated in Diagram 2. The method relies upon tensioning of the reinforcement material so that stresses can be transferred laterally through the reinforcement material. This will act to support placement equipment and reduce the probability of local failures. At the same time, the use of ‘fingers’ reduces global loading and helps prevent failure by this mechanism.
This capping technique is expensive, and best suited for small confined areas of a tailings storage facility with very low shear strengths such as decant pond areas.
Radial Telescopic Conveyor Capping
Radial Telescopic Conveyor Capping can provide an alternative to placing an initial confining layer using earthmoving equipment without the need to place machinery or personnel over the tailings surface where the storage is narrow enough to achieve coverage.
This capping approach involves the initial placement of a layer of fill material over the tailings surface using a Radial Telescopic Conveyor. Capping fill material is fed into a hopper which filters oversize particles and places suitable material on a Radial Telescopic Conveyor. The Radial Telescopic Conveyor gradually places the material in an arc formation to the desired layer thickness by rotating from side to side and extending the arm of the telescopic conveyor.
Once a suitable capping layer thickness is achieved in an arc formation out to the limits of the Radial Telescopic Conveyor, the conveyor is moved along the edge of the storage to place the next section of capping material. This process is repeated until complete coverage of the storage is achieved.
This capping approach is particularly suited to narrow storages where coverage is achieved from the perimeter of the storage without the need to traffic over the tailings surface.
Beached Capping Layers
The alternative to placing an initial confining layer using earthmoving equipment is to place a layer of granular fill using methods of hydraulic placement, i.e. “beaching”, as illustrated in Diagram 3.
Slurry formed by a mixture of granular material (e.g. coarse reject, spirals reject sands or fly ash) and water, is discharged across the surface of the tailings. The solids settle out to form a sloping beach, and the water is recovered and recycled. Once a full cover of beached material is achieved, the aim is to construct a final placed capping layer with earthmoving machinery over the top.
The significant advantage of this method is that it provides a safe way for placement of an initial capping layer, without any need to traffic over the tailings until the layer is complete. When used with fly ash, a beached capping layer of significant strength is achieved in a relatively short timeframe to support first capping layer placement using mining equipment.
Placement of Secondary Flocculated Tailings
This technique is similar to the beached capping layer approach, but instead of utilising fly ash, secondary flocculated tailings are hydraulically placed over the existing tailings surface at a controlled rate of filling. A strengthened layer is formed that is capable of supporting the placement of subsequent capping layers using conventional methods.
This method can increase the initial tailings settled density and beach slope, improve water recovery, and enhance tailings shear strength by improving the influence of evaporative drying.
The key to the success of this method is discharging secondary flocculated tailings at a low rate such that the rate of rise of the tailings surface is no more than 1-2 m per year. This maximises the drying and consolidation time and shear strength development for each deposited layer of tailings.
The right capping technique to be deployed for a particular tailings storage facility will depend upon the in-situ conditions, client requirements for final land use and the desired time frame for rehabilitation.
It is dangerous to push fill around the tailings site and hope that all will be well. Careful planning, investigation, design and co-ordination of mine site activities by experienced engineers is essential.
ATC Williams has over 40 years’ experience developing suitable and safe, cost-effective capping solutions to meet the needs of our clients. We have the necessary standards and procedures to ensure the correct rehabilitation strategies for soft coal tailings storage facilities.